Was in The Works in Wimbledon today, they had a large bin full of different types of Sharpie pens all 99p per pack. Only the basic colours though.
From what i could see they had:
Sharpie Fine point 4 colour pack (Black, Green, Blue, Red)http://www.sharpie.com/enGB/Product/Sharpie_Fine_Point_Permanent_Marker.html
Sharpie CD/DVD Permanent Marker (double nibbed) 4 colour pack (Black, Green, Blue, Red)http://www.sharpie.com/enGB/Product/Sharpie_CD_DVD_Permanent_Marker.html
Sharpie CD/DVD Permanent Marker 2 pack Blackhttp://www.sharpie.com/enGB/Product/Sharpie_CD_DVD_Permanent_Marker.html
Sharpie Professional Chisel Tip Permanent Marker 3 colour pack (Black, Blue, Red)http://www.sharpie.com/enGB/Product/Sharpie_Professional_Chisel_Tip_Permanent_Marker.html
Sharpie Professional Chisel Tip Permanent Marker 2 pack Blackhttp://www.sharpie.com/enGB/Product/Sharpie_Professional_Chisel_Tip_Permanent_Marker.html
Sharpie Mini Permanent Marker 4 colour pack (Black, Green, Blue, Red)http://www.sharpie.com/enGB/Product/Sharpie_Mini_Permanent_Marker.html
This is the cheapest I've seen them anywhere.
I love Sharpies and bought several packs of each.
The lady was not able to say for sure if it was nationwide but she thought it would be.
Offline only but check for you nearest The Works on the link.